The present invention relates in particular to an articulated stinger for a pipelaying vessel and more particularly to an articulated stinger having flexible joints, which may be used conventionally for S-method pipelaying or vertically for J-method pipelaying.
Along with the increasing development of oil and gas wells offshore, demand for pipelaying apparatus to connect underwater facilities or to bring oil and gas to shore has increased sharply. Special purpose pipelay barges or other floating vessels are employed to lay underwater pipelines along the ocean floor. A typical pipelaying operation involves assembling the pipeline by adding one section of pipe at a time on the barge and then moving the barge ahead as the assembled pipeline is paid out and laid onto the ocean floor. Alternatively, the pipeline may be preassembled and wound onto a rotatable reel which has been mounted on the barge or ship. The pipeline then is spooled off the reel and laid onto the sea floor after the barge or ship arrives at the proper field location.
Any pipeline is capable of withstanding some bending but such bending must stay within predetermined limits to avoid permanently deforming (buckling) or fatiguing (fracturing) the pipeline. As the pipeline is laid from a conventional pipelay barge, the pipeline exits the barge and follows a more or less "S" shaped configuration from the barge deck to the ocean floor. In relatively shallow waters, the vertical descent of the unsupported pipeline, being small, results in a large radius of curvature of the pipeline as it comes off the barge, and the pipe remains safe from bending damage. However, as the water depth increases, the length of unsupported pipeline increases, causing the pipeline to sag under its own weight. Thus, the radius of curvature of the pipeline as it leaves the barge becomes smaller and the bending moments imposed on the pipeline may well exceed the allowable limits and result in permanent deformation of the pipeline.
For deeper water pipelaying operations, a pipe discharge ramp, or so-called "stinger", is used to control the bending of the pipe in the overbend portion of the suspended pipe span. This "stinger" generally is a long, slender, rigid structure which extends outward and curves downward from the stern end of the pipelay barge, and contains rollers for supporting and paying out the pipe as the barge moves forward. The stinger often is used in conjunction with flotation means to force the pipeline to assume a curvature whose minimum radius is great enough to avoid undue bending. Reference may be had to the following patents and literature exemplifying the state of the art: an article by C. G. Langner entitled "The Articulated Stinger: A New Tool for Laying Offshore Pipelines", Offshore Technology Conference, Paper OTC-1073, May, 1969 and U.S. Pat. Nos. 3,331,212; 3,517,519; 3,555,835; 3,538,712; 3,559,413; 3,641,779; 3,670,511; 3,685,305; 3,704,596; 3,736,760; 3,739,590; 3,822,559; 3,911,689; 3,922,870; 3,990,259; 3,994,140; 4,112,698; RE 28,922; and RE 29,591.
However, the current state of the art does not deal satisfactorily with several problems. Thus, as the pipeline leaves the barge and acts against the ramp, it exerts more than just a vertical component of force against the stinger. If the barge deviates slightly from its course or if lateral currents are present which act against the pipeline as it is being laid, the pipeline will attempt to move laterally off the stinger, but since it is held in place, it will exert a sideways or lateral force against the remote end of the stinger. This lateral force is translated into a sideways bending moment at the connection of the stinger to the barge. Even further, the pitching and rolling motions of the vessel may introduce additional bending and torsional moments in the stinger. Such forces, acting on a substantially rigid structure, may reach such a magnitude that would significantly damage the stinger, thereby causing a cessation in the pipelaying operation and down time to repair the stinger or even replace it. And, in the event of using a vertical stinger, which is a particular subject of the present invention, problems with vessel roll are even greater.
As pipelaying progresses into even deeper waters, the slope of the suspended pipeline will become steeper and steeper, and in very deep waters the suspended pipeline will of necessity approach the pipelay vessel in a nearly vertical orientation. Conventional S-method pipelaying in such depths will require a stinger that is extremely long and vulnerable to rough sea conditions. A more satisfactory method of laying pipelines in very deep waters is the J-method, wherein the pipeline is assembled on the barge or ship in a vertical or near-vertical orientation, thereby eliminating the pipe overbend associated with the S-method. Pipe assembly utilizing either a vertical derrick or an adjustably inclined ramp or tower, would require only a relatively small vertical stinger which would control bending of the pipe due to horizontal tension, ocean currents, and pitch and roll motions of the vessel.
Hence, there exists a need for an articulated stinger which not only will accommodate the operating conditions encountered during conventional pipelaying operations without causing damage to the stinger, but also in less conventional pipelaying operations as more particularly described hereinafter.